Tailoring Acid-Salt Hybrid Electrolyte Structure for Stable Proton Storage at Ultralow Temperature

The critical challenges in developing ultralow-temperature proton-based energy storage systems are enhancing the diffusion kinetics of charge carriers and inhibiting water-triggered interfacial side reactions between electrolytes and electrodes. Here an acid-salt hybrid electrolyte with a stable anion-cation-H₂O solvation structure that realizes unconventional proton transport at ultralow temperature is shown, which is crucial for electrodes and devices to achieve high rate-capacity and stable interface compatibility with electrodes. Through multiscale simulations and experimental investigations in the electrolyte employing ZnCl₂ introduced into 0.2 M H₂SO₄ solution, it is discovered that unique anion-cation-H₂O solvation structure endows the electrolyte with low-temperature-adaptive feature and favorable water network channels for rapid proton transport. In situ XRD and multiple spectroscopic techniques further reveal that the stable 3D network structure inhibits free water-triggered deleterious electrode structure distortion by immobilizing free water molecules to achieve outstanding cycling stability. Hence, VHCF//α-MoO₃ hybrid proton capacitors deliver an unexpected capacity of 39.8 mAh g^-1 at a high current density of 1 A g^-1 (-80 °C) and steady power supply under ultralow temperatures (96% capacity retention after 1500 cycles at -80 °C). The anti-freezing hybrid electrolyte design provides an effective strategy to improve the application of energy storage devices in ultralow temperatures.